replace recently added attempt to use visibility macros with one based on gcc guidelines

I can see no reason to define the "C API Call Type", but perhaps it really is needed for some reason.

I have tested this on Linux only.

libs/panners/vbap/wscript

@@ -23,6 +23,7 @@ def build(bld):
     obj.source = [ 'vbap_speakers.cc', 'vbap.cc'  ]
     obj.export_includes = ['.']
     obj.cxxflags     = '-DPACKAGE="libardour_panvbap"'
+    obj.defines      = [ 'ARDOURPANNER_DLL_EXPORTS' ]
     obj.includes     = ['.']
     obj.name         = 'libardour_panvbap'
     obj.target       = 'panvbap'